In refrigeration evaporator systems the refrigerant leaves a condenser as a slightly sub-cooled liquid at a high temperature and pressure. Before the refrigerant enters the evaporator the pressure of the refrigerant has to be brought down to the evaporating pressure and temperature by expanding the refrigerant. In this process a part of the refrigerant vaporises. The energy released by the cooling/expansion is absorbed by the evaporating refrigerant.
The evaporators and condensers used in these systems is often plate heat exchangers comprising a plate pack made of a number of assembled heat transfer plates forming between them interspaces. In most cases, every second plate interspace communicates with a first inlet channel and a first outlet channel, each plate interspace being adapted to define a flow area and to conduct a flow of a first fluid between said inlet and outlet channels. Correspondingly, the other plate interspaces communicate with a second outlet channel for a flow of a second fluid. Thus the plates are in contact with one fluid through one of their side surfaces and with the other fluid through the other side surface, which allows a considerable heat exchange between the two fluids.
Plate heat exchangers of today have heat transfer plates, which in most cases are made of sheet metal blanks which have been pressed and punched to obtain their final shape. Each heat transfer plate is usually provided with at least four ports consisting of through holes punched at the four corners of the plate. The ports of the different plates define said inlet and outlet channels, which extend through the plate heat exchanger transversely of the plane of the plates. Sealing means are arranged around some of the ports in every second plate interspace, round the other ports so as to form two separate channels for the first and the second fluid, respectively. The sealing could be performed by means of gaskets, welding or brazing.
Since considerable fluid pressure levels are obtained in the heat exchanger during operation, the plates need to be sufficiently rigid in order not to be deformed by the fluid pressure. The use of plates made of sheet metal blanks is possible only if the plates are somehow supported. This is usually achieved by the heat transfer plates being formed with some kind of corrugation so that they bear against each other at a large number of points.
The plates may be clamped together between two flexurally rigid end plates or frame plates in a frame and thus form rigid units with flow channels in every plate interspace. The end plates are clamped against each other by means of a number of clam bolts which engage both plates in holes along the circumference of each end plate. Some plate heat exchangers are joined by welding or soldering, wherein the end plates protect the heat transfer plates of the heat exchanger.
Refrigerant evaporators are classified according to how the expansion is arranged. The circulation evaporator may be a thermosiphon, pump or ejector. The two-phase mixture that leaves the expansion valve separates into vapour and liquid in a separator. The liquid mixes with circulating liquid from the evaporator and once more enters the evaporator. The vapour mixes with the vapour from the evaporator and leaves for the compressor.
This type of evaporator always operates with much less than 100% evaporation. The heating surface is thus always wetted by the refrigerant. The heat transfer coefficient is high, thus requiring only a small heat transfer area, but a separator is necessary.
The separator in a flooded system has one or more of the following important functions:                To separate liquid droplets from the vapour        To accumulate the refrigerant content of the system during a shut down.        To even out changes of volume in the system during load variations.        Under certain conditions, the refrigerant may foam and space has to be provided.        To provide a static liquid level which then provides the driving force for the circulation or the suction head for a circulation pump.        The liquid level is also used to control the expansion valve. This is sometimes made in the high-pressure receiver, sometimes in the low-pressure receiver.        To act as an oil trap/separator.        
The separation of the vapour and the liquid is obtained by gravitational forces, sometimes assisted by centrifugal forces, which allow the heavier liquid droplets to settle.
Accordingly, liquid droplets small enough to be kept in suspension by molecular movements, Brownian movements, do not separate. In practice, liquid droplets sometimes much larger than Brownian droplets do not separate either, but there are additional methods to separate them.
Basically there are two types of separators, the horizontal and the vertical type and a hybrid type separator. The horizontal separator has the following properties:                The flow is horizontal. If the residence time is sufficiently long, the droplets separate regardless the velocity.        The hold up time and the height of the separation space determine the efficiency.        When the liquid level increases, the cross section decreases, the velocity, increases and the hold up time decreases, i.e. a decreased separation.        It is easy to connect two or more evaporators or an evaporator with double exits. Double entrances reduce the velocity by 50% but also the separation distance preserving the efficiency.        
A vertical separator has the following properties:                The vapour flow is mainly upwards. If the velocity is lower than the separation velocity the droplets separate. Variation of the liquid content causes a correspondingly large variation of the liquid level.        The liquid level does not affect the vapour velocity.        The liquid body is easily agitated, providing a variable signal to the TEV and a difficult oil separation.        It occupies little floor space, but more head space.        The vertical separator should not be mixed with the cyclone. The cyclone separates particles or droplets by entering the vapour tangentially at a high velocity, thereby creating a strong centrifugal force, which effects the actual separation. Separation is very effective, but the pressure drop is also very high.        It is difficult to connect two or more evaporators or an evaporator with double exits.        
A hybrid separator has the properties:                It is basically a horizontal separator with a vertical vessel attached to the bottom.        The liquid level is maintained in this vessel. Separation occurs in the horizontal part. The velocity is independent of the liquid level.        The total refrigerant filling is less than the horizontal.        The liquid level is less affected by the flashing refrigerant or the circulating vapour-liquid mixture.        
A traditional separator is made of carbon steel and is short and bulky with a large diameter. However, this construction adds a considerable weight to an evaporator and the size and shape thereof give rise to problems concerning height restrictions, etc. The cost for a short and wide separator is also much higher than for an elongated and slender separator.
A traditional separator with its weight is held up by external support constructions with space allowances over the separator making the length of connecting pipes from the evaporator to the separator relatively long.
Moreover, the separator normally contains a mass of refrigerant that is hazardous, e.g. for reasons of toxicity, flammability, decomposition, etc.